The Planck likelihoods fit parameters describing foreground power
spectrum templates and instrumental nuisance parameters at the
Monte-Carlo Markov Chain (MCMC) sampling stageThroughout this paper we use the CosmoMC sampler [1]} developed and maintained by Antony Lewis
(https://cosmologist.info.cosmomc). The version of CosmoMC is almost identical to that used in PCP18.. This approach has been adopted by ground-based
experiments [2]}, [3]}. The foreground/nuisance model used for the Planck likelihoods has been described in
described in previous Planck papers PPL13, PPL15 and PPL18. For
completeness, in this section we summarize the model adopted in this paper,
detailing changes that we have made to the model since PCP18.
